Real Estate Assets and Construction in Progress (Summary of Properties Owned or Leased by the Company) (Details) $ in Thousands	Sep. 30, 2017 USD ($) property	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($) property
Real Estate Properties [Line Items]
Land	$ 86,192		$ 74,130
Buildings and improvements	1,668,503		1,524,767
Construction in progress	429,390		365,960
Total cost	$ 2,184,085		$ 1,964,857
Number of facilities leased | property	13		13
Duluth, Georgia | Other Assets.
Real Estate Properties [Line Items]
Land		$ 1,900
Buildings and improvements		8,700
Construction in progress		$ 100
Owned Properties | Atlanta, Georgia (Atlanta-Metro)
Real Estate Properties [Line Items]
Land	$ 20,416		$ 15,397
Buildings and improvements	450,128		434,965
Construction in progress	24,269		32,422
Total cost	494,813		482,784
Owned Properties | Richmond, Virginia
Real Estate Properties [Line Items]
Land	2,180		2,180
Buildings and improvements	253,346		237,347
Construction in progress	61,257		70,580
Total cost	316,783		310,107
Owned Properties | Irving Texas
Real Estate Properties [Line Items]
Land	8,606		8,606
Buildings and improvements	259,999		204,713
Construction in progress	68,478		69,653
Total cost	337,083		282,972
Owned Properties | Suwanee, Georgia (Atlanta-Suwanee)
Real Estate Properties [Line Items]
Land	3,521		3,521
Buildings and improvements	169,625		171,376
Construction in progress	2,876		2,013
Total cost	176,022		176,910
Owned Properties | Chicago, Illinois
Real Estate Properties [Line Items]
Land	9,400		9,400
Buildings and improvements	80,737		45,848
Construction in progress	125,350		100,623
Total cost	215,487		155,871
Owned Properties | Piscataway New Jersey
Real Estate Properties [Line Items]
Land	7,466		7,466
Buildings and improvements	82,571		82,210
Construction in progress	36,250		17,261
Total cost	126,287		106,937
Owned Properties | Santa Clara, California
Real Estate Properties [Line Items]
Buildings and improvements	100,565		98,708
Construction in progress	6,909		7,078
Total cost	107,474		105,786
Owned Properties | Sacramento, California
Real Estate Properties [Line Items]
Land	1,481		1,481
Buildings and improvements	62,529		62,102
Construction in progress	1,868		390
Total cost	65,878		63,973
Owned Properties | Princeton, New Jersey
Real Estate Properties [Line Items]
Land	20,700		20,700
Buildings and improvements	32,823		32,788
Construction in progress	469		538
Total cost	53,992		54,026
Owned Properties | Fort Worth, Texas
Real Estate Properties [Line Items]
Land	9,079		136
Buildings and improvements	17,587		610
Construction in progress	32,611		49,116
Total cost	59,277		49,862
Owned Properties | Ashburn, Virginia
Real Estate Properties [Line Items]
Construction in progress	32,829
Total cost	32,829
Owned Properties | Phoenix, Arizona
Real Estate Properties [Line Items]
Construction in progress	25,091
Total cost	25,091
Owned Properties | Other
Real Estate Properties [Line Items]
Land	2,213		4,113
Buildings and improvements	35,541		37,810
Construction in progress	133		6,283
Total cost	37,887		48,206
Leased Properties | Owned Properties
Real Estate Properties [Line Items]
Land	1,130
Buildings and improvements	123,052
Construction in progress	11,000
Total cost	$ 135,182
Leased Properties | Owned Properties | Princeton, New Jersey
Real Estate Properties [Line Items]
Land			1,130
Buildings and improvements			116,290
Construction in progress			10,003
Total cost			$ 127,423